Add dependent: :destroy to Page#elements relations

When I delete a page, I do expect its elements to be gone afterwards,
too.

app/models/alchemy/page/page_elements.rb

@@ -23,6 +23,10 @@ module Page::PageElements
         -> { order(:position).fixed.available },
         class_name: 'Alchemy::Element',
         inverse_of: :page
+      has_many :dependent_destroyable_elements,
+        -> { not_nested },
+        class_name: 'Alchemy::Element',
+        dependent: :destroy
       has_many :contents, through: :elements
       has_and_belongs_to_many :to_be_swept_elements, -> { distinct },
         class_name: 'Alchemy::Element',

spec/models/alchemy/page_spec.rb

@@ -338,6 +338,14 @@ module Alchemy
           expect(news_page.elements.pluck(:name)).to include('contactform')
         end
       end
+
+      context 'destruction' do
+        let!(:page) { create(:alchemy_page, autogenerate_elements: true) }
+
+        it 'destroys elements along with itself' do
+          expect { page.destroy! }.to change(Alchemy::Element, :count)
+        end
+      end
     end
 
     # ClassMethods (a-z)